1. Field of the Invention
The present invention relates to an airbag device for the driver's seat for expanding an airbag so that a driver can be protected during a collision of a vehicle, wherein the airbag device for the driver's seat is mounted by the upper part of a boss of the center of the steering wheel.
2. Description of Related Art
In a conventional airbag device for a driver's seat described in JP-A-2003-170804, an airbag is stored in a folded condition by the upper part of a boss of the center of a steering wheel. Furthermore, the stored airbag is held by a bag holder and covered with an openable door part of an airbag cover.
The airbag comprises a driver-side wall part disposed toward the driver at a top side when expansion is completed, and a carbody-side wall part at the lower side of the airbag facing the steering wheel. Both the driver-side wall part and the carbody-side wall part are in an approximate disk shape. The airbag is formed by coupling the outer edges of the driver-side wall part and the carbody-side wall part to each other. In the airbag, a gas opening is disposed near the center of the carbody-side wall part, and the periphery of the gas opening is made the attachment region to the bag holder. An inflator that can supply expansion gas is inserted into the gas opening.
The airbag is stored in a way that when it is folded, first, the carbody-side wall part and the driver-side wall part are overlapped and spread flat, and then folded so that widths between sides of the outer peripheries in a diameter direction with the gas opening as the center is reduced. When the expansion gas is supplied, the airbag pushes and opens the door part, and then expands.
Furthermore, in the conventional airbag device for the driver's seat, the folding process of the airbag for reducing the widths was performed in at least two stages where the directions in which the size is reduced are perpendicular to each other. In the folding process of the first stage, areas by two edge portions are folded toward the driver-side so as to overlap.
Therefore, when the airbag spreads and expands, unfolding of the folding of the first-step was restricted due to friction between lapped regions in the first-stage. Unfolding of the folding of the second stage was performed quickly at all positions. As a result, the airbag was able to spread out in a predetermined direction (the lateral direction of the unfolding of the folding in the second-stage) when unfolding the folding of the second stage.
Thus, the folding had an advantage over the folding as described in JP-A-11-48890 in that the airbag can be spread and expanded in a manner such that unnecessary projection toward the driver is restricted. In addition, a certain level of thickness is secured, and spreading width in a predetermined direction can be secured in the initial stage of expansion of the airbag. However, in conventional folding as described in JP-A-11-48890, the areas by two edge portions of an airbag that was spread flat were simply wound up toward the driver-side wall and brought close to a gas opening side.
The conventional airbag device for driver's seat described in JP-A-2003-170804 has regions which overlap in the folding process of the first-step. The overlapping regions are disposed above the gas opening, and therefore unfolding of these regions is delayed. Therefore, in the initial stage of expansion of the airbag, the thickness of a region of the airbag over the gas opening, in other words, the region near the center of the airbag when the expansion is completed or the region of the airbag near the center of the steering wheel, is small. Therefore, there has been room for improvement in the conventional airbag device for the driver's seat with regard to securing thickness in the initial stage of expansion near the portion which will be the center when expansion of the airbag is completed.